How would you prevent making Cisco Email Security Appliance (ESA) an opened relay?

How would you prevent making Cisco Email Security Appliance (ESA) an opened relay?
A . Make sure to configure ESA RAT (Recipient Access Table) of incoming Mail Listener (Public listener) so it accepts emails for all the domains ESA is responsible for and rejects emails towards all other recipient domains
B . Handle this by specific configuration of incoming Mail Flow policy
C . Include all of your email domains into HAT table into one of the WHITELIST Sender Groups of your Public Listener
D . Make sure ESA Private Listener has appropriate external sender domains explicitly included in RELAYUIST in Private Listeners HAT (Host Access Table)

View Answer

Answer: A